Energy recovery devices that regenerate hydraulic fluid energy by driving a hydraulic motor with return hydraulic fluid returning from a swing hydraulic motor, performing power generation by an electric motor directly connected to the hydraulic motor, and storing the generated electric energy in a battery are described in Patent Literatures 1 and 2, for example.
Further, Patent Literature 2 describes a method for controlling the tilting angle of a regenerative hydraulic motor such that pressure necessary for the braking of the swing hydraulic motor is maintained at the time of the hydraulic fluid regeneration, as a control method for the energy recovery device.